Method and system for forming absorber layer on metal coated glass for photovoltaic devices

ABSTRACT

An apparatus for forming a solar cell includes a housing defining a vacuum chamber, a rotatable substrate support, at least one inner heater and at least one outer heater. The substrate support is inside the vacuum chamber configured to hold a substrate. The at least one inner heater is between a center of the vacuum chamber and the substrate support, and is configured to heat a back surface of a substrate on the substrate support. The at least one outer heater is between an outer surface of the vacuum chamber and the substrate support, and is configured to heat a front surface of a substrate on the substrate support.

FIELD

The disclosure relates to photovoltaic devices generally, and moreparticularly relates to a system and method for producing photovoltaicdevices.

BACKGROUND

Photovoltaic devices (also referred to as solar cells) absorb sun lightand convert light energy into electricity. Photovoltaic devices andmanufacturing methods therefor are continually evolving to providehigher conversion efficiency with thinner designs.

Thin film solar cells are based on one or more layers of thin films ofphotovoltaic materials deposited on a substrate such as glass. The filmthickness of the photovoltaic materials ranges from several nanometersto tens of micrometers. Such photovoltaic materials function as lightabsorbers. A photovoltaic device can further comprise other thin filmssuch as a buffer layer, a back contact layer, and a front contact layer.

Copper indium gallium diselenide (CIGS) is a commonly used absorberlayer in thin film solar cells. CIGS thin film solar cells have achievedexcellent conversion efficiency (>20%) in laboratory environments. Mostconventional CIGS deposition is done by one of two techniques:co-evaporation or selenization. Co-evaporation involves simultaneouslyevaporating copper, indium, gallium and selenium. The different meltingpoints of the four elements makes controlling the formation of astoichiometric compound on a large substrate very difficult.Additionally, film adhesion is very poor when using co-evaporation.Selenization involves a two-step process. First, a copper, gallium, andindium precursor is sputtered on to a substrate. Second, selenizationoccurs by a reaction of the precursor with H₂Se/H₂S at 500° C. or above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read in conjunction with the accompanying drawings. Itis emphasized that, according to common practice, the various featuresof the drawings are not necessarily to scale. On the contrary, thedimensions of the various features are arbitrarily expanded or reducedfor clarity. Like reference numerals denote like features throughoutspecification and drawing.

FIG. 1 is a plan view of an apparatus for forming a solar cell, whichcomprises at least one inner heater and at least one outer heater, inaccordance with some embodiments.

FIG. 2 is a plan view of an apparatus for forming a solar cell, whichcomprises at least three sputtering sources, at least one evaporationsource, at least one inner heater and at least one outer heater, inaccordance with some embodiments.

FIGS. 3A and 3B illustrate examples of a portion of the substratesupport comprising a plurality of metal posts and a plurality of metalframes, which is configured to hold a respective substrate, inaccordance with some embodiments.

FIGS. 4A and 4B illustrate a plan view and a cross-sectional view of asubstrate held by metal frames along its entire length in accordancewith some embodiments.

FIGS. 5A-5C illustrate a plan view and two cross-sectional view of anexample of a substrate attached with metal frames contacting along aportion of its length through at least one fixture on the same side ofthe metal frames, in accordance with some embodiments.

FIGS. 6A-6C illustrate a plan view and two cross-sectional view of anexample of a substrate attached with metal frames contacting along aportion of its length through at least one fixture on both sides of themetal frames, in accordance with some embodiments.

FIG. 7A is a top view illustrating an example of configuration with atleast one inner heater and at least one outer heater which areconfigured to heat a substrate from both its back and front surface insome embodiments.

FIG. 7B is a magnified cross-section view of a portion of the substratein FIG. 7A, showing a metal layer coated on a glass substrate, inaccordance with some embodiments.

FIG. 8 is a schematic view of a portion of an example of apparatus,showing that one or more additional heaters are coupled to metal framesor metal posts in the rotatable substrate support, in accordance withsome embodiments.

FIG. 9 is a flow chart diagram illustrating an example of method forfabricating solar cells comprising heating a substrate simultaneouslyfrom both its front surface and its back surface, in accordance withsome embodiments.

FIG. 10 is a flow chart diagram illustrating an example of method forforming absorber layer over the front surface of a substrate in someembodiments.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. In the description, relativeterms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,”“below,” “up,” “down,” “top” and “bottom” as well as derivative thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should beconstrued to refer to the orientation as then described or as shown inthe drawing under discussion. These relative terms are for convenienceof description and do not require that the apparatus be constructed oroperated in a particular orientation. Terms concerning attachments,coupling and the like, such as “connected” and “interconnected,” referto a relationship wherein structures are secured or attached to oneanother either directly or indirectly through intervening structures, aswell as both movable or rigid attachments or relationships, unlessexpressly described otherwise.

Glass such as soda lime glass can be used as a substrate in thin filmsolar cells. In general, a back contact layer, a light absorber layer ofphotovoltaic material, a buffer layer, and a front contact layer can bedeposited over the substrate, respectively. Examples of suitablematerials for the back contact layer deposited over the glass include,but are not limited to copper, nickel, molybdenum (Mo) or any othermetal or conductive material. Example of suitable materials for thelight absorber layer include but are not limited to cadmium telluride(CdTe), copper indium gallium selenide (CIGS) and amorphous silicon(α-Si). Depending on the type of the photovoltaic material for theabsorb layer, the buffer layer can be either an n-type or a p-typesemiconductor materials including but are not limited to CdS and ZnS.The front contact layer is a transparent conductive material such asindium tin oxide (ITO).

The inventors have determined that non-uniformity of temperature cancause a substrate—particularly a metal coated glass substrate—to deformand even crack when a subsequent layer is deposited over the substrateunder heating conditions. The factors causing such a temperaturevariation include but are not limited to material difference between thesubstrate and the back contact layer; and material difference betweenthe substrate and substrate holder contacting the substrate. Theinventors have determined that a metal layer coated on glass as the backcontact layer can reflect heat while glass absorbs heat inside thevacuum chamber, and can cause uneven temperature distribution cross thesubstrate. The substrate holder and other structural materials insidethe vacuum chamber are generally metals. Different coefficient ofthermal expansion (CTE) can worsen the problem to cause deformation,warping, cracking or other damage to the substrate.

In some embodiments, this disclosure provides an apparatus and a methodfor forming a solar cell, in which a substrate such as glass is heatedwithout cracking or deformation, and an absorbed layer can be formedover the substrate. For example, in accordance with some embodiments inthis disclosure, a good absorber layer can be deposited over a metalcoated glass substrate when the substrate is heated under a temperatureclose to its glass transition temperature. The apparatus and the methodare suitable for forming a solar cell or other photovoltaic devicesusing a substrate such as glass of different sizes including large glasspanel.

Unless expressly indicated otherwise, references to a “front side” of asubstrate made in this disclosure will be understood to encompass theside on which a light absorber layer will be deposited. References to a“back side” of the substrate made below will be understood to encompassthe other side opposite to the side where the light absorber layer willbe deposited. References to a “substrate” will be understood toencompass a substrate with or without a back contact layer, for example,a metal coated glass substrate. When the substrate is a metal coatedglass, the “back side” is the glass layer while the “front side” is themetal layer deposited over the glass layer as the back contact layer.

FIG. 1 is a plan view of an apparatus 100 for forming a solar cell. Inaccordance with some embodiments, apparatus 100 comprises a housingdefining a vacuum chamber 105, a rotatable substrate support 120, atleast one inner heater 117 and at least outer heater 118.

As shown, apparatus 100 includes a housing 105 defining a vacuumchamber. In some embodiments, housing 105 may be shaped as a polygon.For example, as shown in FIG. 1, housing 105 may be octagonally shaped.In some embodiments, housing 105 has one or more removable doors builton one or more sides of the vacuum chamber. Housing defining a vacuumchamber 105 may be composed of stainless steel or other metals andalloys. For example, housing 105 can define a single vacuum chamberhaving a height of approximately 2.4 m (2.3 m to 2.5 m) with a lengthand width of approximately 9.8 m (9.7 m to 9.9 m). References to a“vacuum chamber” 105 or a “housing” 105 defining a vacuum chamber inthis disclosure will be understood to encompass the same meaning.

Apparatus 100 includes a rotatable substrate support 120. In someembodiments, substrate support 120 is configured to hold a substrate 130on each of a plurality of surfaces 122 facing an interior surface of thevacuum chamber. In some embodiments, examples of suitable materials forsubstrate 130 include but are not limited to glass and metal coatedglass. In some embodiments, rotatable substrate support 120 is shaped asa polygon. For example, as shown in FIG. 1, a plurality of substrates130 are held on a plurality of surfaces 122 in a substantially octagonalshaped rotatable substrate support 120. Any other suitable shape, suchas rectangle or square, can be used for rotatable substrate support 120.

Rotatable substrate support 120 comprises a plurality of metal posts121, and a plurality of metal frames 124 connected to the plurality ofmetal posts 121 in some embodiments. Each of the plurality of metalframes 124 may also comprise at least one fixture 123 in the edge. Metalposts 121 and metal frames 124 are configured to hold a respectivesubstrate 130.

FIGS. 3A and 3B illustrate examples of a portion of the substratesupport 120 comprising a plurality of metal posts 121 and a plurality ofmetal frames 124, which is configured to hold a respective substrate130, in accordance with some embodiments. Substrate 130 can be in anyshape. In some embodiments, metal frames 124 are configured to holdsubstrate 130 in rectangular shape. As shown in FIG. 3A, metal frame 124can be in one piece holding one respective substrate 130 in the middle.As shown in FIG. 3B, substrate can be held with multiple pieces of metalframes 124. Examples of suitable materials for metal frames 124 andmetal posts 121 can be any metal. In some embodiments, metal frames 124and metal posts 121 are made of a material selected from the groupconsisting of stainless steel, titanium and molybdenum.

Metal posts 121 and metal frames 124 can be configured in differentcombinations to hold substrate 130. In some embodiments, each of theplurality of metal frames 124 is configured to support a respectivesubstrate 130 along its entire length, and each substrate edge is heldalong its entire length through the at least one fixture on theplurality of metal frames 124. FIGS. 4A and 4B illustrate a plan viewand a cross-sectional view of a substrate held by metal frame 124 alongits entire length.

In other embodiments, each respective substrate 130 is attached with arespective one of metal frames 124 contacting along a portion of itslength, and each substrate edge is retained by a respective fixture 123at selected points on the respective metal frame 124. FIGS. 5A-5Cillustrate a plan view and two cross-sectional views of an example ofsubstrate 130 attached with metal frames 124 contacting along a portionof its length through at least one fixture on the same side of metalframes 124. FIGS. 5B and 5C illustrate a cross section along the sectionline 5B-5B and 5C-5C shown in FIG. 5A, respectively. In this example,all the fixtures 123, are on one side of metal frame 124. FIGS. 6A-6Cillustrate a plan view and two cross-sectional views of another exampleof a substrate 130 attached with metal frames 124 contacting along aportion of its length through at least one fixture on both sides of themetal frames. FIGS. 6B and 6C illustrate a cross section along thesection line 6B-6B and 6C-6C shown in FIG. 6A, respectively. In thisexample, some of fixtures 123 are on one side of metal frame 124, whilesome other fixtures 123 are on the other side of metal frame 124. Onepiece of metal frame 124 is shown in FIGS. 4A-4B, 5A-5C, and 6A-6C forpurpose of illustration. Rotatable substrate support 120 may comprise aplurality of metal frames 124.

Referring back to FIG. 1, in some embodiments, substrate support 120 isrotatable about an axis in the vacuum chamber. FIG. 1 illustrates aclockwise direction of rotation for rotatable substrate support 120. Insome embodiments, substrate support 120 is configured to rotate in acounter-clockwise direction. In some embodiments, rotatable substratesupport 120 is operatively coupled to a drive shaft, a motor or othermechanism that actuates rotation from a surface of the vacuum chamber.Substrate support 120 can be rotated at a speed, for example, betweenapproximately 5 and 100 RPM (e.g., 3 and 105 RPM). In some embodiments,a speed of rotation is selected to minimize excessive deposition ofabsorption components over substrate 130. In some embodiments, substratesupport 120 rotates at a speed of approximately 80 RPM (e.g. 75-85 RPM).

In some embodiments, apparatus 100 also comprises a rotatable drum 110,which is disposed within vacuum chamber 105 and coupled to a top orbottom surface of vacuum chamber 105. Rotatable drum 110 may alsocomprise supporting beams 111 connected with rotatable substrate support120. Rotatable drum 110 can be operatively coupled to substrate support120 through support beams 111, and is configured to rotate substratesupport 120 inside vacuum chamber 105. In some embodiments, rotatabledrum 110 has a shape that is substantially the same as the shape ofsubstrate support 120. In other embodiments, rotatable drum 110 can haveany suitable shape.

In some embodiments, apparatus 100 also comprises at least one innerheater 117 and at least one outer heater 118. In some embodiments, theat least one inner heater 117 is between a center of vacuum chamber 105and substrate holder 120, and is configured to face a back surface ofsubstrate 130 and heat a back surface of substrate 130 on substratesupport 120. In some embodiments, the at least one inner substrate canthe whole substrate 130 from the back surface. In some embodiments, theat last one inner heater 117 is configured to heat a plurality ofsubstrates 130 held on rotatable substrate support 120 when substratesupport 120 is rotated. The at least one inner heater 117 can have anysuitable shape. In some embodiments, the rotatable substrate support 120is polygonally-shaped, and the at least one inner heater 117 isconfigured to have a circular shape to avoid any collision betweensubstrate support 120 and the at least inner heater 117. In otherembodiments the at least one inner heater 117 has a shape that issubstantially the same as the shape of substrate support 120. In someembodiments, the rotatable substrate support 120 is polygonally-shaped,and the at least one inner heater 117 has a shape substantially the sameas the shape of substrate support 120. For example, as shown in FIG. 1,the at least one inner heater 117 has a substantially octagonalarrangement.

In some embodiments, the at least one inner heater 117 is disposed tomaintain a substantially uniform distance about the perimeter ofsubstrate support 120. In some embodiments, the at least one innerheater 117 is disposed between an interior surface of rotatablesubstrate support 120 and rotatable drum 110. A power source of the atleast one inner heater 117 can extend through a surface of rotatabledrum 110. In some embodiments, substrate support 120 is rotatable aroundthe at least one inner heater 117. In some embodiments, the at least oneinner heater 117 can be coupled to a top or bottom surface of vacuumchamber 105. The at least one inner heater 117 can be rotatable. Inother embodiments, the at least one inner heater 117 is not rotatable.The at least one inner heater 117 can include, but is not limited to,infrared heaters, halogen bulb heaters, resistive heaters, or anysuitable heater for heating a substrate 130 during a deposition process.In some embodiments, the at least one inner heater 117 can heat asubstrate to a temperature between approximately 295° C. and 655° C.(e.g. 300° C. and 650° C.).

The at least one outer heater 118 is located between an outer surface(or shell) of vacuum chamber 105 (housing) and substrate support 120,and is configured to heat a front surface of substrate 130 on substratesupport 120, in accordance with some embodiments. In some embodiments,the at least one outer heater 118 is attached on the interior surface ofvacuum chamber 105. The at least one outer heater 118 can be configuredto heat substrate 130 from a front surface of substrate 130 duringrotation of substrate support 120.

The at least one outer heater 118 can include, but is not limited to,infrared heaters, halogen bulb heaters, resistive heaters, or anysuitable heater for heating a substrate 130 during a deposition process.In some embodiments, the at least one outer heater 118 can heat asubstrate 130 to a temperature between approximately 295° C. and 655° C.(e.g. 300° C. and 650° C.).

In some embodiments, the at least one outer heater 118 is configured toheat substrate 130 simultaneously while the substrate is heated by theat least one inner heater 117. The heat emitted from the at least oneouter heater 118 mitigates non-uniformity of temperature within sample130. For example, referring to FIGS. 7A-7B, when substrate 130 is apiece of glass with the front side coated with metal as the back contactlayer of a solar cell, the metal layer may reflect heat from the atleast one inner heater 117 to the glass layer. FIG. 7A is a top viewillustrating an example with at least one inner heater 117 and at leastone outer heater 118 which are configured to heat a substrate 130 fromboth its back and front surface. FIG. 7B is a magnified cross-sectionview of a portion of substrate 130 in FIG. 7A. As shown in FIG. 7B, thefront surface of glass substrate is coated with a metal layer 124, whichwill be the back contact layer of a solar cell. The back surface 122 ofsubstrate 130 is glass. Metal layer 124 reflects the heat from the backsurface 122 which will absorb such extra heat. The at least one outerheater 118 can be configured to generate additional heat to maintainuniform temperature distribution across substrate 130. This example isused for illustration purpose only. The at least one outer heater 118can be used in other ways.

In some embodiments, the at least inner heater 117 or the at least oneouter heater 118 is configured to heat substrate 130 according to apredetermined heating profile. For example, a stepwise heating profilewith various heating rates can be used in some embodiments. In someembodiments, both the at least one inner heater 117 and the at least oneouter heater 118 is configured to operate according to a predeterminedheating profile. In some embodiments, the heating profile comprisesheating the substrate from room temperature to final high temperaturestep by step instead of straight heating. Each step of heating can be100° C. interval and stay at that temperature for 5 minutes. Thus thetemperature difference between substrate center and edge can beminimized. In some embodiments, the at least one inner heater 117 isconfigured to be operated according to a program which providesautomatic adjustment according to actual temperature of a sample 130during a process.

Apparatus 100 also comprises a cooling element 115 in some embodiments.Cooling element 115 is configured to cool down the temperature insidevacuum chamber 105, for example, between two operations, or before a newplurality of substrates 130 is loaded onto rotatable substrate support120. For example, temperature of inner chamber wall can reach atemperature up to 700° C. after a deposition process. Cooling element115 is a coil system having a coolant such as water in some embodiments.Apparatus 100 can be also configured to introduce conductive cooling gassuch as nitrogen inside vacuum chamber 105, in accordance with someembodiments of this disclosure. The method of cooling using gas insidevacuum chamber 105 and the method using coil cooling with a coolant canbe used separately or simultaneously.

In some embodiments, apparatus 100 further comprises one or moreadditional heaters 119 coupled to the plurality of metal frames 124 ormetal posts 121 in rotatable substrate support 120. The additionalheaters 119 are configured to rotate with the substrate support 120 andheat a respective substrate 130 during rotation. Examples of anadditional heater 119 include, but are not limited to, infrared heaters,halogen bulb heaters, resistive heaters, or any suitable heater forheating a substrate 130 during a deposition process. Additional heaters119 can be lamp heaters configured to emit infra-red radiation in someembodiments.

FIG. 8 is a schematic view of a portion of an example of rotatablesubstrate support 120 in apparatus 100. FIG. 8 shows that one or moreadditional heaters 119 are coupled to metal frames 124 or metal posts121 in rotatable substrate support 120, in accordance with someembodiments. An additional heater 119 can be provided on each edge of arespective metal frame 124 or a metal post 121. In some embodiments,additional heaters are only on a top or bottom edge of a metal frame 124or a metal post 121. In some embodiments, additional heaters 119 areonly on side edges of a metal frame 124 or a metal post 121. In someembodiments, additional heaters 119 are on both top or bottom edges andside edges of a metal frame 124 or a metal post 121. In someembodiments, different combinations of additional heaters 119 areconfigured to provide different heating zones. The heat emitted fromadditional heaters 119 can be used to offset heat absorbed by metalposts 121 or metal frames 124. Additional heaters 119 can also beoperated according to a predetermined heating profile or a computerprogram providing automatic response to actual temperature on substrate130 inside vacuum chamber 105. Additional heaters 119 can be coordinatedwith the at least one inner heater 117 and the at least one outer heater118 during a process.

Referring back to FIG. 1, in some embodiments, apparatus 100 furthercomprises at least one sputtering source 135, at least one evaporationsource 140 and at least one isolation pump 152. Each of the at least onesputtering source 135 is configured to deposit a respective firstingredient. In some embodiments, the respective first ingredient is oneingredient of an absorber layer over a front surface of substrate 130.The at least one evaporation source 140 is disposed in vacuum chamber105 and configured to deposit a second ingredient. In some embodiments,the second ingredient is one ingredient of the absorber layer over thefront surface of substrate 130. Each of the least one isolation pump 152is configured to prevent materials from the at least one evaporationsource 140 from contaminating the at least one sputtering source 135. Arespective isolation pump 152 is disposed between each of the at leastone sputtering source 135 and an adjacent one of the at least oneevaporation source 140 in some embodiments.

The at least one sputtering source 135 can be disposed on the housingdefining vacuum chamber 105. Sputtering source 135 can be, for example,a magnetron, an ion beam source, a RF generator, or any suitablesputtering source configured to deposit a respective first ingredientfor an absorber layer over the front surface of substrates 130. Eachsputtering source 135 includes at least one sputtering target 137.Sputtering source 135 can utilize a sputtering gas. In some embodiments,sputtering is performed with an argon gas. Other possible sputteringgases include krypton, xenon, neon, and similarly inert gases.

As shown in FIG. 1, apparatus 100 can include at least two sputteringsources 135. Each sputtering source 135, having at least one sputteringtarget 137, is configured to deposit a portion of a respective firstingredient for an absorber layer over a front surface of substrate 130.Each respective first ingredient for the absorber layer can have adifferent chemical composition. In some embodiments, the at least twosputtering sources 135 are disposed adjacent to each other. In someother embodiments, the at least two sputtering sources 135 are disposedin two locations spaced apart from each other. For example, in FIG. 1, afirst and second sputtering source 135 are disposed on opposing sides ofvacuum chamber 105 and substantially equidistant from evaporation source140 about the perimeter of vacuum chamber 105.

In some embodiments, a first sputtering source 135 is configured todeposit atoms of a first type (e.g. copper (Cu)) in the first ingredientfor absorber layer over at least a portion of a surface of substrate130. A second sputtering source 135 is configured to deposit atoms of asecond type (e.g. indium (In)) in the first ingredient for absorberlayer over at least a portion of a surface of substrate 130. In someembodiments, the first sputtering source 135 is configured to depositatoms of a first type (e.g. copper (Cu)) and a third type (e.g. gallium(Ga)) in the first ingredient for absorber layer over at least a portionof substrate 130. In some embodiments, a first sputtering source 135includes one or more copper-gallium sputtering targets 137 and a secondsputtering source 135 includes one or more indium sputtering targets137. For example, a first sputtering source 135 can include twocopper-gallium sputtering targets and a second sputtering source 135 caninclude two indium sputtering targets. In some embodiments, acopper-gallium sputtering target 137 includes a material ofapproximately 70 to 80% (e.g. 69.5 to 80.5%) copper and approximately 20to 30% (e.g. 19.5 to 30.5%) gallium. In some embodiments, apparatus 100has a first copper-gallium sputtering target 137 at a first copper:gallium concentration and a second copper-gallium sputtering target 137at a second copper: gallium concentration for grade compositionsputtering. For example, a first copper-gallium sputtering target caninclude a material of 65% copper and 35% gallium to control monolayerdeposition to a first gradient gallium concentration and a secondcopper-gallium sputtering target can include a material of 85% copperand 15% gallium to control monolayer deposition to a second gradientgallium concentration. The plurality of sputtering targets 137 can beany suitable size. For example, the plurality of sputtering targets 137can be approximately 15 cm wide (e.g. 14-16 cm) and approximately 1.9 mtall (e.g. 1-8-2.0 m).

In some embodiments, a sputtering source 135 that is configured todeposit a plurality of absorber layer atoms of indium over at least aportion of each substrate 130 can be doped with sodium (Na). Forexample, an indium sputtering target 137 of a sputtering source 135 canbe doped with sodium (Na) elements. Doping an indium sputtering target137 with sodium may avoid or minimize an alkali-silicate layer in thesolar cell. This improvement may result in lower manufacturing costs forthe solar cell as sodium is directly introduced to the absorber layer.In some embodiments, a sputtering source 135 is a sodium-doped coppersource having between approximately two and ten percent sodium (e.g.1.95 to 10.1 percent sodium). In some embodiments, an indium sputteringsource 135 can be doped with other alkali elements such as, for example,potassium. In other embodiments, apparatus 100 can include multiplecopper-gallium sputtering sources 135 and multiple sodium doped indiumsputtering sources 135. For example, apparatus 100 can have a 65:35copper-gallium sputtering source 135 and an 85:15 copper-galliumsputtering source 135 for grade composition sputtering.

In some embodiments, an evaporation source 140 is configured to deposita second ingredient of the absorber layer over at least a portion ofeach substrate 130. In some embodiments, the second ingredient of theabsorber layer comprises selenium, and can include any suitableevaporation source material. In some embodiments, evaporation source 140is configured to produce a vapor of such an evaporation source material.The vapor can condense upon substrate 130. For example, evaporationsource 140 can be an evaporation boat, crucible, filament coil, electronbeam evaporation source, or any suitable evaporation source 140. In someembodiments, evaporation source 140 is disposed in a first sub-chamberof vacuum chamber 105. In some embodiments, the vapor of source materialcan be ionized, for example using an ionization discharger, prior tocondensation over substrate 130 to increase reactivity.

An isolation pump 152 is configured to prevent materials from the atleast one evaporation source 140 from contaminating the at least onesputtering source 135. In some embodiments, a respective isolation pump152 is disposed between each of the at least one sputtering source 135and an adjacent one of the at least one evaporation source 140. In theembodiment of FIG. 1, isolation pumps 152 are vacuum pumps. In someembodiments (not shown), one or more of the isolation pumps 152 isconfigured to maintain the pressure in an evaporator source 140sub-chamber (not shown) lower than the pressure in vacuum chamber 105.Isolation pumps 152 can be configured to evacuate absorber layerparticles of the second ingredient (emitted by evaporation source 240)from vacuum chamber 105, prevent diffusion of these particles into thesputtering targets 237, and prevent these particles from contaminatingthe two sputtering sources 235.

In embodiments including a plurality of sputtering sources 135 and/or aplurality of evaporation sources 140, apparatus 100 can include aplurality of isolation sources to isolate each of the evaporationsources from each of the sputtering sources 135. For example, inembodiments having first and second sputtering sources 135 disposed onopposing sides of a vacuum chamber and an evaporation source 140disposed there between on a perimeter of the vacuum chamber 105,apparatus 100 can include a first isolation pump 152 disposed betweenthe first sputtering source 135 and evaporation source 140 and a secondisolation pump 152 disposed between the second sputtering source 135 andevaporation source 140. In the illustrated embodiment, apparatus 100includes an isolation pump 152 disposed between evaporation source 140and one of the two sputtering sources 135.

As shown in FIG. 1, apparatus 100 can include an isolation baffle 170disposed about the evaporation source 140. Isolation baffle 170 can beconfigured to direct a vapor of an evaporation source material to aparticular portion of substrate 130. Isolation baffle 170 can beconfigured to direct a vapor of an evaporation source material away froma sputtering source 135. Apparatus 100 can include an isolation baffle170 in addition to one or more isolation sources to minimize evaporationsource material 122 contamination of one or more sputtering sources 135.Isolation baffle 170 can be composed of a material such as, for example,stainless steel or other similar metals and metal alloys. In someembodiments, isolation baffle 170 is disposable. In other embodiments,isolation baffle 170 is cleanable.

In some embodiments, apparatus 100 can include a loading/unloadingsubstrate chamber 182, buffer chamber 155, post-treatment chamber 180and unload lock 184. In various embodiments, post-treatment chamber 180can be configured for post treatment of the solar cell such as, forexample, cooling the solar cell.

In some embodiments, apparatus 100 can include one or more in-situmonitoring devices (not shown) to monitor process parameters such astemperature, chamber pressure, film thickness, or any suitable processparameter.

Apparatus 100 of FIG. 1 can also be used to form solar cells ofdifferent absorber films, for example, a copper-zinc-tin-sulfur-selenium(CZTSS) absorber film. In some embodiments, a number of CZTSS absorberlayer are formed in apparatus 100 by further providing tin, copper,zinc, or copper/zinc targets, as targets 137. Evaporation source 140 mayuse sulfur, selenium or both sulfur and selenium as source material.Additionally, another evaporation source 140 may be used to separatelyprovide selenium and sulfur source material.

FIG. 2 is a plan view of an apparatus 200 for forming a solar cell inaccordance with some embodiments. Apparatus 200 comprises at least two(e.g., three) sputtering sources 135, at least one evaporation source140, at least one inner heater and at least one outer heater. In FIG. 2,like items are indicated by like reference numerals, and for brevity,descriptions of the structure, provided above with reference to FIG. 1,are not repeated. The at least two (e.g., three) sputtering sources 135are configured to deposit at least three different compositions as arespective first ingredient for an absorber layer over a front surfaceof substrate 130. For example, a first sputtering source 135 isconfigured to deposit copper and gallium from a first target 137comprising copper and/or gallium. A second sputtering source 135 isconfigured to deposit indium from a first target 137 comprising indium.In some embodiments, copper and gallium can be deposited in twodifferent first sputtering sources 135.

FIG. 9 is a flow chart diagram illustrating an example of method 900 forfabricating solar cells comprising heating a substrate simultaneouslyfrom both its front surface and its back surface, in accordance withsome embodiments.

In step 910 of this exemplary method 900, a substrate 130 is firstprovided and secured on a rotatable substrate support 120 inside avacuum chamber 105 in an apparatus for forming solar cells. For example,the apparatus 100 or 200 described above can be used. Substrate 130 hasa front surface and a back surface. Examples of substrate 130 andconfigurations for holding substrate 130 are described in FIG. 1.Rotatable substrate support 120 is in a polygon shape and comprises aplurality of metal posts 121 and metal frames 124 in some embodiments.Metal posts 121 and metal frames are configured to hold substrate 130,as described in FIGS. 3A-3B, 4A-4B, 5A-5C and 6A-6C. The front surfaceof substrate 130 is disposed facing an interior surface of vacuumchamber 105 in some embodiments, as shown in FIGS. 7A-7B.

In step 920, substrate support 120 is rotated. In some embodiments,substrate support 120 is continuously rotated at a certain speed asdescribed in FIG. 1. In some embodiments, substrate support 120 isintermittently rotated.

Step 930 comprises heating substrate 130 simultaneously using at leastone inner heater 117 and at least one outer heater 118 as described inFIG. 1. The inner heaters 117 faces a back surface of the substrate 130and heats substrate 130 from the back surface of substrate 130. The atleast one outer heater is configured to heat a front surface of thesubstrate during rotation in some embodiments. In some embodiments,substrate 130 is simultaneously heated using the at least one innerheater 117 and the at least one outer heater 118, according to apredetermined temperature profile. In some embodiments, the at least oneinner heater 117 or at least one outer heater 118, or both, heatsubstrate according to a predetermined temperature profile as describedin FIG. 1. In Some embodiments, the predetermined temperature profile isused for minimizing temperature difference between the substrates and aplurality of metal posts or metal frames in the rotatable substratesupport. In some embodiments, the at least one inner heater 117 or atleast one outer heater 118 or both are operated based on a programproviding automatic response based on the actual temperature ofsubstrate 130 inside vacuum chamber 105.

Step 936 is used in some embodiments, and is omitted from otherembodiments. In some embodiments, the method 900 further comprisesheating substrate 130 using one or more additional heaters 119 coupledto the plurality of metal frames 124 or metal posts 121 in rotatablesubstrate support 120. As described in FIG. 1, examples of additionalheaters 119 include, but are not limited to, lamp heaters emittinginfrared radiation, for example, IR lamps. The one or more additionalheaters 119 rotate with substrate support 120 in some embodiments.

In step 940, an absorber layer is formed over the front surface ofsubstrate 130 as described above. In some embodiments, the step offorming an absorber layer over the front surface of substrate 130 (instep 940) includes depositing a respective first ingredient for anabsorber layer over the front surface of substrate 130 from at least onesputtering source 135; and depositing a second ingredient of theabsorber layer over the front surface of substrate 130 from anevaporation source 140 disposed in the vacuum chamber 105. In depositingthe second ingredient, at least one isolation pump 152 as described inFIG. 1, is used to prevent materials from the at least one evaporationsource 140 from contaminating the at least one sputtering source 135.

FIG. 10 is a flow chart diagram illustrating an example of a method forforming an absorber layer over the front surface of a substrate. In someembodiments, substrate 130 comprises a glass layer in the back surfaceand a metal layer deposited over the glass layer on the front surface.The step of depositing a respective first ingredient for an absorberlayer (in step 940 of FIG. 9) comprises at least three steps in theexample of FIG. 10. These include: depositing copper and gallium from afirst sputtering source 135 (step 942); and depositing indium from asecond sputtering source 135 (step 944). The step of depositing thesecond ingredient of the absorber layer comprises depositing seleniumfrom an evaporation source 140 in step 948.

Referring back to FIG. 9, in some embodiments, the method also comprisesstep 950 of cooling substrate 130 and vacuum chamber 105 with an inertconductive gas after forming an absorber layer, or any other layers ofsolar cells.

This disclosure provides an apparatus and a method for forming a solarcell. In accordance with some embodiments, the apparatus for forming asolar cell comprises a housing defining a vacuum chamber; a rotatablesubstrate support, at least one inner heater and at least outer heater.The rotatable substrate support inside the vacuum chamber is configuredto hold a substrate. The at least one inner heater is between a centerof the vacuum chamber and the substrate support, and is configured toheat back surface of a substrate on the substrate support. The at leastone outer heater is located between the housing and the substratesupport, and is configured to heat a front surface of a substrate on thesubstrate support. In some embodiments, the at least one outer heater isattached on the interior surface of the vacuum chamber. In someembodiments, the apparatus also comprises a rotatable drum configured torotate the substrate support inside the vacuum chamber. In someembodiments, the apparatus of this disclosure further comprises at leastone sputtering source, at least one evaporation source and at least oneisolation pump. In some embodiments, the apparatus further comprises oneor more additional heaters coupled to the plurality of metal frames ormetal posts in the rotatable substrate support. The additional heatersare configured to rotate with the substrate support and heat arespective substrate during rotation.

In accordance with some embodiments, this disclosure provides anapparatus for forming a solar cell, comprising a vacuum chamber, arotatable substrate support, a rotatable drum, at least one sputteringsource, at least one evaporation source, at least one inner heater andat least one outer heater. The rotatable substrate support inside thevacuum chamber is of a polygonal shape configured to hold at least onesubstrate. The rotatable drum is disposed within the vacuum chamber. Theat least one sputtering source is configured to deposit a respectivefirst ingredient, for example, a respective first ingredient for anabsorber layer over a front surface of the substrate. The at least oneevaporation source is disposed in the vacuum chamber and configured todeposit a second ingredient, for example, a second ingredient for theabsorber layer over the front surface of the substrate. Each innerheater faces a back surface of a respective substrate, and is configuredto heat that substrate. The at least one outer heater is disposedbetween an outer surface (shell) of the vacuum chamber and the rotatablesubstrate support, is configured to heat a front surface of the at leastone substrate during rotation. In some embodiments, the apparatusfurther comprises one or more additional heaters coupled to a pluralityof metal frames or metal posts in the rotatable substrate support. Theone or more additional heaters are configured to rotate with thesubstrate support and heat a respective substrate during rotation. Insome embodiments, the apparatus also comprises at least one isolationpump. The at least one isolation pump is disposed between each of the atleast one sputtering source and an adjacent one of the at least oneevaporation source, and is configured to prevent materials from theevaporation source from contaminating the sputtering source.

This disclosure also provides a method for forming a solar cell. Asubstrate is first provided and secured on a rotatable substrate supportinside a vacuum chamber. The front surface of a substrate is disposedfacing an interior surface of the vacuum chamber. The method furthercomprises rotating the substrate support; heating the substratesimultaneously using at least one inner heater and at least one outerheater; and forming an absorber layer over the front surface of thesubstrate. The inner heater faces a back surface of the substrate andheats the substrate from the back surface of the substrate. The at leastone outer heater is configured to heat a front surface of the substrateduring rotation. The forming an absorber layer over the front surface ofthe substrate includes depositing a respective first ingredient for anabsorber layer over the front surface of the substrate from at least onesputtering source; and depositing a second ingredient of the absorberlayer over the front surface of the substrate from an evaporation sourcedisposed in the vacuum chamber. In some embodiments, the substratecomprises a glass layer coated with a metal layer on the front surface.The depositing a respective first ingredient for an absorber layercomprises depositing copper, gallium and indium from at least twodifferent sputtering sources, respectively. The depositing the secondingredient of the absorber layer comprises depositing selenium from theevaporation source.

In some embodiments, the method further comprises heating the substrateusing one or more additional heaters coupled to the plurality of metalframes or metal posts in the rotatable substrate support. The one ormore additional heaters rotate with the substrate support. In someembodiments, the method also comprises cooling the substrate and thevacuum chamber with an inert conductive gas after forming an absorberlayer.

Although the subject matter has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly, to include other variants and embodiments,which may be made by those skilled in the art.

What is claimed is:
 1. An apparatus for forming a solar cell,comprising: a housing defining a vacuum chamber; a rotatable substratesupport inside the vacuum chamber configured to hold a substrate,wherein the rotatable substrate support comprises a plurality of metalposts and a plurality of metal frames connected to the plurality ofmetal posts, each of the plurality of metal frames comprising at leastone fixture, the plurality of metal posts and the plurality of metalframes configured to hold a respective substrate; at least one innerheater configured to heat a back surface of a substrate on the substratesupport; at least one outer heater between the housing and the substratesupport, the at least one outer heater configured to heat a frontsurface of a substrate on the substrate support; and one or moreadditional heaters coupled to the plurality of metal frames or metalposts in the rotatable substrate support, and configured to rotate withthe substrate support and heat a respective substrate during rotation.2. The apparatus of claim 1, wherein the plurality of metal frames areconfigured to hold rectangular substrates, and the plurality of metalframes and the plurality of metal posts are made of a material selectedfrom the group consisting of stainless steel, titanium and molybdenum.3. The apparatus of claim 1, wherein each of the plurality of metalframes is configured to support a respective substrate along its entirelength, and each substrate edge is held along its entire length throughthe at least one fixture on the plurality of metal frames.
 4. Theapparatus of claim 1, wherein each respective substrate is attached witha respective one of the metal frames contacting along a portion of itslength, and each substrate edge is retained by a respective fixture atselected points on the respective metal frame.
 5. The apparatus of claim1, wherein the one or more additional heaters are lamp heatersconfigured to emit infra-red radiation.
 6. The apparatus of claim 1,wherein the rotatable substrate support is polygonally-shaped, and theat least one inner heater is configured to have circular shape to avoidcollision between the substrate support and the at least one innerheater during the rotation.
 7. The apparatus of claim 1, furthercomprising: a rotatable drum disposed within the vacuum chamber andcoupled to a top or bottom surface of the vacuum chamber, the rotatabledrum having a shape that is substantially the same as the shape of thesubstrate support and comprising supporting beams connected with therotatable substrate support, wherein the rotatable drum is configured torotate the substrate support inside the vacuum chamber.
 8. The apparatusof claim 1, wherein the at least one outer heater is attached on theinterior surface of the vacuum chamber.
 9. The apparatus of claim 1,further comprising: at least one sputtering source, each of the at leastone sputtering source configured to deposit a respective firstingredient for an absorber layer over a front surface of the substrate;at least one evaporation source disposed in the vacuum chamber andconfigured to deposit a second ingredient of the absorber layer over thefront surface of the substrate; and at least one isolation pump, each ofthe least one isolation pump configured to prevent materials from the atleast one evaporation source from contaminating the at least onesputtering source, wherein a respective isolation pump is disposedbetween each of the at least one sputtering source and an adjacent oneof the at least one evaporation source.
 10. An apparatus for forming asolar cell, comprising: a vacuum chamber; a rotatable substrate supportof a polygonal shape inside the vacuum chamber configured to hold atleast one substrate; a rotatable drum disposed within the vacuumchamber, the rotatable drum having a polygonal shape, comprisingsupporting beams; at least one sputtering source, configured to deposita respective first ingredient; at least one evaporation source in thevacuum chamber and configured to deposit a second ingredient; at leastone inner heater, each of the at least one inner heater configured toheat a back surface of a respective substrate on the substrate support;and at least one outer heater between a shell of the vacuum chamber andthe rotatable substrate support, the at least one outer heaterconfigured to heat a front surface of a substrate on the substratesupport.
 11. The apparatus of claim 10, wherein: the rotatable substratesupport comprises a plurality of metal posts and a plurality of metalframes attached with the plurality of metal posts, each of the pluralityof metal frames comprising at least one fixture, the plurality of metalposts and the plurality of metal frames configured in a shape that issubstantially the same as the shape of the substrate.
 12. The apparatusof claim 11, further comprising: one or more additional heaters coupledto the plurality of metal frames or metal posts in the rotatablesubstrate support, and configured to rotate with the substrate supportand heat a respective substrate during rotation.
 13. The apparatus ofclaim 12, wherein the one or more additional heaters are lamp heatersconfigured to emit infra-red radiation.
 14. The apparatus of claim 10,wherein the at least one outer heater is attached on the interiorsurface of the vacuum chamber.
 15. An apparatus for forming a solarcell, comprising: a housing defining a vacuum chamber; a rotatablesubstrate support inside the vacuum chamber configured to hold asubstrate; at least one inner heater configured to heat a back surfaceof a substrate on the substrate support; at least one outer heaterbetween a shell of the vacuum chamber and the substrate support, the atleast one outer heater configured to heat a front surface of thesubstrate on the substrate support; at least one sputtering source, eachof the at least one sputtering source configured to deposit a respectivefirst ingredient for an absorber layer over a front surface of thesubstrate; and at least one evaporation source disposed in the vacuumchamber and configured to deposit a second ingredient of the absorberlayer over the front surface of the substrate.
 16. The apparatus ofclaim 15, further comprising: one or more additional heaters coupled tothe plurality of metal frames or metal posts in the rotatable substratesupport, and configured to rotate with the substrate support and heat arespective substrate during rotation.
 17. The apparatus of claim 16,wherein the one or more additional heaters are lamp heaters configuredto emit infra-red radiation; and the at least one outer heater isattached on the interior surface of the vacuum chamber.
 18. Theapparatus of claim 15, further comprising: at least one isolation pump,each of the least one isolation pump configured to prevent materialsfrom the at least one evaporation source from contaminating the at leastone sputtering source, wherein a respective isolation pump is disposedbetween each of the at least one sputtering source and an adjacent oneof the at least one evaporation source.